Collagen assembly into extracellular matrices will be studied during morphogenesis, growth, and repair. Tendon, cornea and dermal fibroblasts establish a hierarchy of extracellular spaces associated with fibrils, bundles, and tissue specific aggregates during development. The distinct domains provide a mechanism for the fibroblast to influence the extracellular steps in matrix assembly. A discontinuous fibrillar matrix is assembled, and fibril segments, about 10 microm long, are deposited into bundles by the 14-day chick embryo tendon fibroblast. We propose that the fibril segment is a normal assembly intermediate, permitting orderly linear, lateral, and intercalary development as well as growth. The size, structure, and fate of fibril segments during different stages of development will be defined using electron microscopy, serial sections, and three dimensional reconstruction techniques. Fibril segments at different stages of development also will be extracted, morphologically characterized, analyzed chemically, and these data correlated with those obtained in situ. In early stages of development, segments are extractable as discrete units while at later stages this may not be the case. We hypothesize that a linear and/or lateral fusion of segments may be responsible for longer, thicker, more mature fibrils. Changes in the fibril surface may mediate these fusions and the ability to chemically analyze segments will permit the identifi- cation of differences related to development or tissue source. The assembly of segments followed by a post- depositional maturation may be an important general mechanism. To test this, fibril segments will be studied during morphogenesis of the chick cornea and dermis, as well as during repair. Constituents of the extracellular compartments will be localized immunochemically. Changes in the surface properties of the segments and/or the interfibrillar matrix may be related to compositional differences in the compartments resulting in the addition or removal of specific proteoglycans, collagen types, propeptides or other macromolecules. Understanding the mechanisms involved in collagen fibril formation, deposition, and the development of connective tissue architecture in situ will contribute to our understanding of development, growth, injury and repair as well as inherited disorders.